DESCRIPTION: Chronic myelogenous leukemia (CML) is a disorder in which the initial lesion is known to occur at the level of hemopoietic stem cells (HSCs). The leukemic cells of over 95 percent of CML patients harbor the Philadelphia (Ph), which occurred as the result of a reciprocal translocation between chromosomes 9 and 22. At the molecular level, this translocation generates the bcr/abl hybrid gene, whose product has elevated tyrosine kinase activity. These changes are very similar to those observed in a similarly altered and activated v-abl oncogene discovered in the mouse. The introduction of either the bcr/abl or v-abl oncogene into hemopoietic stem/progenitor cells and after their engraftment into recipient mice could produce a disease resembling CML in humans. Over the past few years, several investigators, including the principal investigator, have actively employed a mouse model to understand disease pathogenesis. To take it further, they are establishing a gene therapy mouse model for CML. By using fetal liver cells as a source of HSCs, they have shown that transplantable leukemia in mice can be developed after retroviral transfer of either bcr/abl or v-abl oncogene into these fetal HSCs. Furthermore, this mouse model has several aspects that resemble those in humans: (a) untransduced normal HSCs are also present in leukemic mice, and (b) expression of the activated abl oncogene in primitive stem cells is low in Ph +ve human multipotent stem/progenitor cells. In this application, the investigators wish to dissect the model further and to examine if retroviral gene transfer of the anti-bcr/abl sequences into leukemic HSCs would result in retardation or elimination of leukemia development in secondary recipients after retransplantation of single or few leukemic donor HSCs. The specific aims are (1) to determine the proportion of bcr/abl transduced and untransduced long-term repopulating HSCs in leukemic mice; (2) to examine whether untransduced gp105 +ive HSCs have a competitive growth advantage over bcr/abl transduced gp105 +ive HSCs; (3) to verify the effectiveness of the antisense vector in vitro on the suppression of bcr/abl-mediated transformation or leukemia development; (4) to test the efficacy of the antisense therapy by gene transfer into leukemic HSCs using the in vivo mouse model of leukemogenesis.